1. Field of the Invention
This invention relates to a zoom lens optical unit for high magnification observation of a work piece, and in particular to such an optical unit having a zoom lens drive and magnification encoder which is particularly adapted for use with a video camera.
2. Brief Statement of the Prior Art
Precise measurement and inspection equipment has been disclosed and claimed in various patents, including U.S. Pat. Nos. 4,138,823; 4,084,323; and 4,078,314 which disclose mechanical probes used in a measuring apparatus for determining the dimensions of the work piece. These patented items are particularly intended for use with a coordinate measuring machine.
Some attempts have been made to position closed circuit television cameras on measuring machines and, in particular, attempts have been made to position a standard video camera on coordinate optical measuring machines. A number of difficulties are experienced when attempting to use a video camera in such an application, however, a successful application is shown in U.S. Pat. No. 4,884,878 which discloses a zoom lens with a manual adjustment coupled to a video camera for work piece measurement and inspection.
There is a need to automate the zoom capability of such devices. A number of difficulties are experienced when attempting to provide a motor driven zoom lens in this application. The long focal lengths of high magnification lenses, e.g., microscope lenses, which are used in these probes makes the video camera very sensitive to any instabilities in the video probe mounting, and vibrations induced by a motorized drive cannot be tolerated. Additionally, the magnification power of the lens is not a linear function of its axial extension and conventional digital control and position feedback used with stepping motors is not adequate for indicating the magnification power of the zoom lens during its extension and retraction.